FIG. 10 is a perspective view showing the structure of main part of a conventional outer rotor motor 9.
As shown in FIG. 10, the outer rotor motor 9 comprises a ring-shaped rotor magnet 926 with magnetic poles alternately changing in a circumferential direction, and a ring-shaped yoke 924 covering the radially outer surface of the rotor magnet 926. The outer rotor motor 9 further comprises a stator having a stator core 942 with coils wound around teeth 9422 that extend radially and outwardly in all directions, while facing a radially inner surface 926i of the rotor magnet 926 at their ends.
Patent publications 1 to 3 are prior art documents relating to a conventional outer rotor motor, and disclose a technique of covering the radially outer surface of a rotor magnet with a yoke.
Patent Publication 1: Japanese Patent Publication No. 3580878
Patent Publication 2: Japanese Patent Application Laid-Open No. 2005-198447
Patent Publication 3: Japanese Patent Application Laid-Open No. 2001-244110
Unlike an inner rotor motor, in the outer rotor motor 9, a gap between the radially inner surface 926i of the rotor magnet 926 and the ends of the teeth 9422 is exposed to the outside. Thus, unless particular countermeasures are taken such as provision of a cover for shielding the gap from the outside or reduction of an area of a path leading from the outside to the gap, a foreign material such as iron powder attracted by leakage flux may intrude into the gap in the outer rotor motor 9. Such a foreign material may cause an unusual noise during operation.
Leakage flux generated in the outer rotor motor 9 is discussed with reference to FIGS. 11 and 12. FIG. 11 is a schematic view showing the rotor magnet 926 and the stator core 942 when viewed inwardly in the radial direction. FIG. 12 is a schematic view showing the rotor magnet 926, a yoke 924, the stator core 942 and a coil 944 when viewed in the circumferential direction.
In the outer rotor motor 9, there is leakage flux LM which goes out of one magnetic pole to enter another magnetic pole of an axially upper surface 926u and an axially lower surface 926d of the rotor magnet 926 as shown in FIG. 11. In the outer rotor motor 9, there is also leakage flux LM which goes out of a periphery of the radially inner surface 926i of the rotor magnet 926 to enter the axially upper surface 926u or the axially lower surface 926d of the rotor magnet 926 as shown in FIG. 12. These leakage fluxes LM become noticeable, especially when the width in the axial direction of the rotor magnet 926 is extended to such a degree that the opposite ends of the rotor magnet 926 in the axial direction reach points near coil ends of the coils 944 wound around the teeth 9422.